The present invention relates to novel polymers and, more particularly, to biodegradable polymers which have a pre-determined chirality, to processes of preparing such biodegradable polymers, to heterostereoselective complexes, conjugates and compositions containing same and to uses thereof.
Biodegradable polymers are polymeric materials that undergo chemical, metabolic (by biological processes such as hydrolysis or enzymatic digestion) and/or mechanical breakdown, in a biological environment (e.g., within a body of a living organism, in the presence of microorganisms, and/or in a biological medium). The biodegradability of such polymeric materials renders them highly suitable for constructing environmental-friendly products, and for use in a myriad of medical applications.
Biodegradable polymeric materials have therefore been used for many years in diverse fields, including, for example, agriculture, fishing materials, sanitation, and articles for everyday life (e.g., masks, wet tissues (wipes), underwear, towels, handkerchiefs, kitchen towels and diapers). Medical applications that utilize biodegradable polymers include, for example, orthopedic surgery, including fixation of fractures, bone replacement, cartilage repair, meniscal repair, and fixation of ligament; absorbable devices such as screws, pins, plugs, and plates for orthopedic, oral, and craniofacial surgery, as well as absorbable sutures; scaffolds for tissue engineering; and as drug carriers. The use of biodegradable drug carriers is the most effective route for delivering a drug, since it enables a sustained systemic release of the drug while circumventing the undesired residual impact of the carrier.
Since non-toxicity is an inherent prerequisite for biodegradable polymers that are designed for medical applications, the starting materials, the final product and the optional breakdown products of such biodegradable polymers should be non-toxic and benign. Breakdown products should preferably be also small, water-soluble molecules.
The total degradation time of biodegradable polymers can vary from several days to several years, depending mainly on the chemical structure of the polymer chains, and physical properties such density, surface area and size of the polymer.
When used in medical applications, the biodegradable polymer of choice for the intended use is selected according to its properties. Thus, for example, semi-crystalline polymers (e.g., poly(L-lactic acid)) are typically used in medical devices that require good mechanical properties such as sutures, devices for orthopedic and cardiovascular surgery, and stents. Amorphous polymers, on the other hand (e.g., poly(DL-lactic-co glycolic acid)), are attractive in drug delivery applications, where it is important to have homogenous dispersion of the active species within the polymeric matrix.
The degradation rate of biodegradable polymers is determined by various factors such as the initial molecular weight, the exposed surface area, and the polymer's degree of crystallinity, the quantitative ratio of the monomers, in the case of co-polymers, the chirality of the polymer (racemic mixture versus pure enantiomeric or diastereomeric form), the presence of additives or impurities, the mechanism of degradation (e.g., enzymatic cleavage versus hydrolysis), the implantation site (e.g., subcutaneous tissue versus bone), the actual stress the polymer is subjected to, and even the age of the host subject.
Biodegradability is typically accomplished by synthesizing or using polymers that have hydrolytically unstable linkages in the backbone. Chemical hydrolysis of the hydrolytically unstable backbone is the prevailing mechanism for the polymer's degradation. Biodegradable polymers can be either natural or synthetic. Synthetic polymers commonly used in medical applications include, for example, polyethylene glycol, polyvinyl alcohol, and poly(hydroxypropylmetacrylamide). In addition, natural polymers are also used in medical applications. For instance, dextran, hydroxyethylstarch, albumin and partially hydrolyzed proteins find use in various applications ranging from plasma substitute, to radiopharmaceutical to parenteral nutrition.
In general, synthetic polymers may offer greater advantages than natural materials in that they can be tailored to give a wider range of properties and more predictable lot-to-lot uniformity than can materials from natural sources. Synthetic polymers also represent a more reliable source of raw materials, one free from concerns of infection or immunogenicity.
Methods of preparing polymeric materials are well known in the art. However, synthetic methods that successfully lead to the preparation of polymeric materials that exhibit adequate biodegradability, biocompatibility, hydrophilicity and minimal toxicity for medical use are scarce. The restricted number and variety of biopolymers currently available attest to this.
The presently used biodegradable polymers for medical applications can be classified by their chemical structure as follows:
Polysaccharides, such as starch, cellulose, chitine, chitosan, and alginic acid;
Polypeptides of natural origin, such as gelatin;
Polymers having carbon backbones, such as polyvinyl alcohol and polyvinyl acetate; and
Polymers having a hydrolyzable backbone, such as polyesters, polycaprolactones, polyamides, polyurethanes, polyanhydrides and poly(amide-enamines).
Some commonly used biodegradable polymers are co-polymers, which are composed of various combinations and variations of hydrolysable polymers.
One of the most appealing groups of biodegradable polymers is the family of polyesters, which are characterized by a —O—R—C(═O)— repeating unit, in which the ester bonds are easily hydrolyzed to hydroxy carboxylic acid monomers when placed in aqueous medium.
Polyesters can be synthesized by polycondensation of diols and dicarboxylic acids, by self-polycondensation of hydroxy carboxylic acids, or by ring opening polymerization (ROP) of cyclic esters (lactones), in bulk or in solution.
Polycondensation can be applicable for a variety of combination of monomers, but generally requires high temperature and long reaction time to obtain high molecular weight polymers. In addition, the chain length of the obtained polymers cannot be controlled. If higher molecular weight polymers are needed, the initially obtained polymers can be further cross-linked, by using, for example, diisocianates, bis(amino-ethers), phosgene, phosphate and anhydrides. The most useful monomers for polycondensation are lactic, glycolic, hydroxybutyric and hydroxycaproic acids, polymers of which are known for their long history of safety.
Ring-opening polymerization can be performed only with a limited number of monomers, but can be carried out under milder reaction conditions and produces high molecular weight polymers in short time, as compared to polycondensation polymerization. Furthermore, recent progress in catalysts and initiators for living polymerization has enabled obtaining polyesters of controlled chain length.
A particularly advantageous subfamily of polyesters includes the poly hydroxyalkanoates (PHAs). PHAs are naturally produced by numerous microorganisms as energy reserve materials in the presence of excess carbon source when an essential nutrient, such as nitrogen or phosphorus, is available only in limiting concentrations. PHAs also form a part of depsipeptides, bio-oligomers ubiquitous in nature, which are composed of hydroxy and amino acids linked by amide and ester bonds. Depsipeptides have recently shown to exhibit high therapeutic potential as anticancer, anti-viral, antibacterial, antifungal, anti-clotting, anti-antherogenic and anti-inflammatory agents [Villar-Garea and Esteller, Int. J. Cancer:2004:112:171-178; Sparidans et al., Biomed. Chromatogr. 2004:18: 16-20; and Mayer and Gustafson, Int. J. Cancer: 2003:105, 291-299]. Depsipeptides are regarded, together with amino acids, as components in the natural chiral pool.
PHAs completely degrade into water and carbon dioxide under aerobic conditions, and into methane under anaerobic conditions by a variety of soil, sea, lake and sewage microorganisms. PHAs exhibit a wide variety of mechanical properties, from hard crystalline to elastic, depending on the nature of the monomer units. For example, MCL-PHAs (medium chain length: 6-10) are semi-crystalline elastomers with low melting point, low tensile strength and high elongation to break, and can be used as biodegradable rubber after cross linking. Some PHAs possess properties of thermoplastics.
Poly(3-hydroxybutyrate) (PHB), the most studied PHA, has a molecular weight in the range of 10-3,000 kDa with a polydispersity of about 2, when produced from wild-type bacteria. Its glass transition temperature is near 180° C., and the densities of crystalline and amorphous PHB are 1.26 and 1.18 g/cm3, respectively. Its mechanical properties (e.g., Young's modulus and tensile strength) are close to that of polypropylene though its extension to break is markedly lower than that of polypropylene.
Thus, PHAs offer an attractive alternative to commonly used synthetic biodegradable polymers.
A general structure of PHAs is presented in Formula I below, where the asterisks denote chiral centers (asymmetric carbons, when R differs from hydrogen):

Exemplary PHAs include poly(3-hydroxypropionate)(R=methyl), poly(3-hydroxybutyrate)(R=ethyl), poly(3-hydroxyvalerate)(R=propyl), poly(3-hydroxyhexanoate)(R=butyl), poly(3-hydroxyoxtanoate)(R=pentyl) and poly(3-hydroxydodecanoate)(R=nonyl).
Additional general features of PHAs characterization and biodegradation are described, for example, in Vert and Garreau [Clin Mater 1992; 10:3-8]; Seebach and Fritz [International Journal of Biological Macromolecules 1999:25:217-236]; Brandi et al. [Adv Biochem Eng Biotechnol 1990; 41:77-93]; Bonthrone et al. [FEMS Microbiol Rev 1992; 10:269-78]; and Mergaert et al. [FEMS Microbiol Rev 1992; 10:317-22].
Biodegradable polymers that have one or more stereogenic center(s) (namely, optically active polymers), offer advantageous features when used as carriers for drug delivery. Many biologically active molecules are optically active (chiral), and usually the biological activity can vary greatly depending on the optical purity of the molecule. Much research activity has been focused on the development of technologies allowing access to pure enantiomers. When serving as carriers of chiral (optically active) drugs, biodegradable polymers that have a pre-determined chirality can serve for forming a hetero-stereo complexes with a chiral drug, thus serving as an optically active delivery system.
Hetero-stereo complexation is a new concept in the interaction between complementary optically active polymeric chains which can be utilized for the delivery of, for example, peptides, proteins and other optically active macromolecules. Unlike common delivery systems where the drug powder is physically entrapped in a polymer matrix and the peptide is released by diffusion through the matrix, stereocomplexes form a stereospecific interaction between the optically active drug and the complementary polymer chain. The drug release under physiological conditions is by cleavage of the polymer chain, which reduces the interactions with the drug.
Recently, it has been shown that optically active polylactic acid (PLA)-based polymers form stable hetero-stereo complexes with various peptides. Slager and Domb reported heterocomplexation between D-PLA and L-configured peptides such as the luteinizing hormone-releasing hormone (LHRH) [Eur. J. Pharm. Biopharm. (2004) δ 461-469; Macromolecules (2003), 36, 2999-3000] leuproide (an LHRH nonapeptides analogue) [Biomacromolecules (2003), 4, 1308-1315], vapreotide a cyclic octapeptide somatostatin) analogue) [Advanced Drug Delivery Reviews 55 (2003) 549-583], insulin [Biomaterials 23 (2002) 4389-4396], lysozyme and somastatin [Journal of Controlled Release (2005) 107 (3) 474-483].
Furthermore, biodegradable polymers that have a pre-determined chirality can be used to enhance the permeation of the drug delivery system through various membranes. Thus, it has been reported, for example, that enantioselective transdermal permeation was observed with some chiral excipients, indicating that and t using chiral polymers as drug carriers may assure that only the active drug penetrates the skin [Reddy et al., Crit. Rev Ther Drug Carrier Syst. 2000; 17(4):285-325)].
In addition, when biodegradable polymers are designed to biodegrade via enzymatic cleavage, the chirality of the polymer or of portions thereof, namely, L or D, plays a crucial role in the degradation process. Thus, portions of a biodegradable polymer which has a D chirality would be stable to enzymatic degradation, whereby portions of polymer which have L chirality would be readily susceptible to enzymatic degradation.
The beneficial effect of chiral biodegradable polymers, and the other advantages associated with polyesters in general, and PHAs in particular, have prompt many researchers to study the properties and synthesis of α-hydroxy carboxylic acids, the building blocks of these polymers.
There are numerous examples where the hydroxyl group at the α-position provides an internal control element, facilitating stereoselective transformations of a prochiral functional group. It has been found that chiral α-hydroxy carboxylic esters, acids and their salts, offer versatile synthetic intermediates for the synthesis of polyesters that contain additional chiral centers.
Although α-amino carboxylic acids (amino acids, AA) are considered relatively cheap reagents, there are only a few examples of traditional chemical methods that directly produce α-hydroxy-carboxylic acids or their salts from α-amino carboxylic acids or their salts. The main obstacles of such a transformation generally result from the use of water-sensitive reagents and low temperatures on extremely polar compounds at a relatively unreactive and stereochemically sensitive center.
Thus, for example, α-hydroxy carboxylic acids can be prepared using α-keto carboxylic acids or esters as starting materials. This methodology, however, is limited due to the non-chirality, high cost and limited availability of the starting materials. The chirality of the final product is often induced by a stoichiometric amount of a chiral auxiliary, requiring several synthetic steps and/or a bulky and costly chiral reductant.
Enantioselective reduction of enones can be achieved using a catalytic amount of a chiral oxazaborolidine [Coney, et. al. Tet. Lett., 1990, 31,611]. However, a sequence of four chemical conversions are required in order to transform the initially formed chiral alcohol to a α-hydroxy ester with obvious cost and yield implications.
Enzymatic methods that produce chiral α-hydroxy-carboxylic acids have also been suggested. These methods utilize α-keto carboxylic esters or acids as precursors and involve catalysis by purified or isolated reductases [see, for example, U.S. Pat. Nos. 5,098,841; 4,326,031; and 5,686,275).
WO 02/33110 discloses processes that utilize α-amino carboxylic acids or their salts as starting materials for the efficient and inexpensive production of α-hydroxy acids. This process involves the use of two enzymes: amino acid deaminases (AAD) along with lactate dehydrogenases (LAD). This patent application, however, fails to teach the use of hydroxy carboxylic acid for preparing biodegradable polyesters, and hence further fails to teach the provision of biodegradable polyesters while utilizing amino acids as the starting material.
The use of proteinaceous materials for producing biodegradable polymers has been recently disclosed in WO 04/50732. According to the teachings of WO 04/50732, primary amine and/or amide groups of a proteinaceous substrate are replaced by hydroxyl and/or carboxyl groups, respectively, and the resulting substrate is subjected to polymerization. The hydroxyl and carboxyl groups are introduced to the substrate by reaction with nitrous acid or nitrous oxides. The hydroxyl and carboxyl groups are introduced to the substrate by replacing amines and amides either at the N-terminus of the substrate or at a side chain thereof. Polymerization is thereafter effected via formation of esters bonds between hydroxyl and carboxyl groups. In preferred embodiments of this patent application, amine or amide groups at the side chain of peptides are replaced by hydroxyl or carboxyl groups and polycondensation is effected between two compatible side chains of two polymers, to thereby generate polyamides that are interlinked therebetween via polyester bonds. The conditions under which the modification of the proteinaceous material and the polymerization thereof are effected are relatively harsh and are not directed at maintaining the chirality of the proteinaceous substrate.
Furthermore, the functionality of the proteinaceous material is affected by utilizing its side chain for polymerization. WO 04/50732 therefore fails to teach processes of polymerizing proteinaceous materials while maintaining the original chirality and functionality of the pre-polymerized material upon polymerization.
Thus, there is a widely recognized need for, and it would be highly advantageous to have, novel processes of producing polymers, in particular polyhydroxyalkanoates, in which the chirality and functionality (e.g., side chain structure) of the pre-polymerized monomers forming the polymer, is maintained.